Physics and physical measurement

The realm of Physics, measurements, uncertainties, vectors and scalars

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Order of magnitude kg:

  • mass of universe = 10^50
  • mass of sun = 10^30
  • mass of earth = 10^25
  • mass of a car = 10^3
  • 1 gram = 10^-3
  • mass of proton or neutron = 10^-27
  • mass of electron = 10^-30
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Order of magnitude metres:

  • extent of visible universe = 10^25
  • one light year = 10^16
  • height of person = 10^0
  • wavelength of visible light = 10^-6
  • diameter of a atom = 10^-10
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Order of magnitude seconds:

  • age of the universe = 10^18
  • human life span = 10^9
  • one year = 10^7
  • one day = 10^5
  • period of visible light = 10^-15
  • shortwest lived subatomic article = 10^-23
  • passage of light across the nucleus = 10^-23
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Order of magnitude ratios

Ratio

= 10^-10 / 10^-15

=10^5

= 5 orders of magnitude

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Measurements and uncertainties - fundamental units

length - metre - m

mass - kilogram - kg

time - second - s

electric current - ampere - A

thermodynamic temperature - kelvin - K

amount of substance - mole - mol

luminous intensity - candela - cd

Fundamental units always stay the same, are readily accessible for comparison, are reproducible all over the world so people can check their instruments

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Fundamental Units derivations

Metre - length of path travelled by light in a vacuum in 1/299,792,453 seconds

Kg - mass of a piece of platinum iridium alloy kept in Sevres (France)

Second - time for 9,192,631,770 vibrations of a Ceasium-133 atom

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macro v micro

Macroscopic - visible to the naked eye e.g. a person's height

Microscopic - can be inferred from electron diffraction e.g. diameter of an atom

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Fundamental and derived units

Frequency - f or v - hertz - 1/s

Force - F - Newton - kgms^-2

Work - W - Joule - kg m^2 s^-2 - Nm

Energy - Q - Joule - Nm

Power - P - Watt - kg m^2 s^-3 - Js^-1

Pressure - P - Pascal - kg/ms^-2 - Nm^-2

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Fundamental and derived units

Charge - Q - Coulomb - As

Potential difference - V - Volt - Kg m^2 s^-3 / A - J/C

Resistance - R - Ohm - Kg m^2 s^-3 A^-2 - V/A

Magnetic Field Intensity - B - Tesla - Kg s^-3 /A - N/A/m

Magnetic Flux - Weber - Kg m^2 s^-2 A^-2 - Tm^2

Activity - A - Bequerel - 1/s

Absorbed Dose - W/m - Gray - m^2 s^-2 - J/Kg

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Prefixes - 10^x

3 - kilo - k

6 - mega - M

9 - giga - G

12 - tera - T

15 - peta - P

18 - exa - E

21 - zetta - Z

24 - yotta - Y

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Prefixes - 10^x

-3 - milli

-6 - micro

-9 - nano

-12 - pico

-15 - femto

-18 - atto

-21 - zepto

-24 - yocto

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random and systematic error

Random - Due to variations in the performance of instruments and operator. Reduced by taking repeats and averaging. A precise experiment has low random error.

Systematic - Due to poorly calibrated or bad quality instruments. Consistent error. An accurate experiment has low systematic error.

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Significant figures

An answer cannot have more s.f. than any of the figures in the calculation.

Round off to the correct number of s.f. in the last step of any calculation.

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